SUBJECT: Controlling the pressure in the stuffing box area 4-10
We have very little control over the products that we must seal, but we have a great deal of control over how we elect to seal them. Special seals are seldom the answer. In most cases satisfactory seal life can be obtained by carefully choosing the seal materials for temperature and chemical compatibility, selecting a balanced o- ring seal design and then controlling the environment in the stuffing box to prevent a change in the fluid characteristics from affecting the seal performance.
Controlling stuffing box pressure is one of these controls and is extremely important in many seal applications. In the following paragraphs we will discuss several methods of controlling stuffing box pressure, but first we will learn where the environmental control is necessary.
We can raise the pressure in the stuffing box to :
We can raise the pressure between two seals to :
We can lower the pressure in the stuffing box :
Now to the actual techniques :
In this application we connect a recirculation line from the discharge side of the pump to the stuffing box or a flush connection in the gland. You should install a close fitting bushing into the bottom of the stuffing box with a clearance of .002 inch/ inch (0.002 mm/mm) of shaft diameter. The bushing can be manufactured from any compatible material with the fluid you're sealing. Carbon is often selected as a first choice.
A properly installed balanced o-ring seal will not generate enough heat to flash a liquid between the seal faces as long as the stuffing box pressure is at least one atmosphere (one bar or 15 psi.) higher than the product vapor point. You should have no problem in getting this additional pressure if you've installed the restriction bushing.
The technique is the same as discharge recirculation, but in this application we connect the stuffing box or seal gland to the suction side of the pump or a low pressure sump instead. We do not use a restriction bushing in this application because the differential pressure can cause the bushing to move and contact the mechanical seal. Suction recirculation works best with the proper gland connection, but it can be used with the lantern ring connection if necessary. Be sure the connection is on the bottom or as close to the bottom as possible of the stuffing box or gland.
Caution: Lowering the pressure in the stuffing box is sometimes a bad idea because of the danger of flashing the sealing fluid. The technique is commonly used, however, to remove air that might be trapped in the stuffing box of a vertical pump or to provide normal circulation through the stuffing box when the sealing fluid contains solids. By connecting to the suction side you'll be pulling fluid from behind the impeller, through the stuffing box, and then to the lower pressure on the suction side of the pump. Fluid behind the impeller usually contains less solids than fluid coming from the pump discharge side. It should be mentioned that this technique works very well on most closed impeller pumps and those open impeller designs that adjust to the volute of the pump. Open impeller designs that adjust to the back plate and double ended, single impeller designs have the stuffing box pressure just about at suction pressure so this application doesn't work very well. We go to larger diameter stuffing boxes in those applications.
Another common application for this technique is to cross connect the stuffing boxes of a multi-stage pump to equalize the pressures and balance the seal face wear.
Using two or more seals with a lower pressure between them.
Some seal companies use this as a technique to stage the pressure in a high pressure application. I do not approve of this method because the operator is lead to believe that there are multiple seals in the pump when in fact the multiple seals are acting as one and a failure in any one of the seals will fail them all. I believe you would be better off purchasing a high pressure seal for this application.
Using two seals with a convection tank installed between them.
This is done to take the load off the seal that is sealing a non lubricant or to prevent a pressure drop across the seal faces. The convection tank is filled with a lubricating liquid and the pressure is adjusted to provide the necessary pressure differential. With the proper instrumentation you will be able to tell which seal wears out or fails first. The second seal will act as a back up until you can shut the valves and start the repair.
As an example : In the pump stuffing box you have a 75 psi.(5 bar) non lubricating liquid. You cannot afford product dilution so you install two seals with a lubricating buffer fluid of 75 psi. (5 bar) between them. This will take the load off of the inboard seal that is sealing the non lubricant and shift the load to the outer seal that is containing the lubricating barrier fluid.
The convection tank can be purchased or manufactured from an appropriate corrosion resistant material. Some companies (Coca Cola as an example) ship their product in a similar tank and then scrap the tank because of sanitary or safety regulations. Many of these tanks can be purchased at a low price and modified for your needs. The air connection on the top will allow you to pressurize the tank to the correct pressure for your application.
Keep in mind that convection tank applications are limited by the combination of seal size, face combination, barrier fluid pressure and shaft speed. Check with your seal supplier for a specific recommendation.
When ever possible avoid selecting petroleum base liquids for the barrier fluid circulating in the tank and between the two seals. Petroleum fluids have a very low specific heat that will cause overheating and "coking" problems. If you have the choice. Water is the ideal heat barrier fluid because of its conductivity and high specific heat number. If water is not acceptable choose any compatible fluid with a high conductivity and high specific heat value.
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